A variety of turbomachinery such as gas turbine engines which include axial flow compressors or fans or bypass arrangements utilize projections such as midspan or tip shrouds or other damping means to reduce vibratory loading on blade airfoils. Because adjacent surfaces of such projections or shrouds are in direct contact during engine operation, impact and a type of sliding wear sometimes called adhesive wear occurs at points of contact. It is generally believed that adhesive wear may occur from a combination of impacting and rubbing which produces a repetitive scuffing action of the type produced by vibratory loading during operation of the gas turbine engine. Such adhesive wear can occur between the type of projections mentioned above and the term "projection" is intended to include a variety of protruberances or projections from an airfoil for the purpose of defining at least a portion of a shroud, platform or damping member.
Prior to the present invention the contact surfaces between such members had been provided with a surface means in the form of a coating, typically tungsten carbide in a binder such as cobalt, applied by spray deposition methods. However, during operation of gas turbine engines including such a coating, it had been recognized that undersirable spalling, chipping and wear of such coatings could lead to premature damage to the projection to which it was applied.